1. Field of the Invention
The present invention relates to a method and a system for evaluating an absorption light amount distribution in a resist in photolithography, and evaluating a bottom shape of a resist pattern on a wafer surface.
2. Description of the Related Art
In the technical field of photolithography, there is a demand for a pattern with fineness close to a maximum limit resolution of a reduction-projection type exposing apparatus, with a development in integration density of an LSI. Accordingly, there is a problem of critical dimension control of a resist pattern formed on a wafer. A variation in thickness of a resist film is known as a factor of a variation in dimensions of the resist pattern. The variation in resist film thickness is mainly due to a variation in amount of light absorbed in a resist (hereinafter referred to as "variation in absorption light amount") which is caused by an optical interference effect within a multi-layer structure on a semiconductor substrate including the resist. The dependency of the resist pattern upon the resist film thickness is generally called "swing curve". The dependency of the absorption light amount upon the resist film thickness is also called "swing curve" in general. By contrast, in the prior art, the amount of light absorbed in the resist is calculated, assuming that the light to be absorbed is made incident normal to a multi-layer film including a resist.
However, since the use of an exposing apparatus with a high NA (numerical aperture), a modified illumination and a phase-shifting mask has recently been considered, a possible result based on the aforementioned assumption may differ from an actual result. For example, the papers titled,
"Thin-film interference effects in photolithography for finite numerical aperture" in Journal of Optical Society America A, Vol. 8, No. 1, pp. 123, Douglas A. Bernard, et al., and
"Optimization of optical properties of resist processes" in SPIE Advances in Resist Technology and Processing, VIII Vol. 1466, p. 297, T. A. Brunner, show, based on calculations, that in accordance with the angle of incidence of light on the resist, the absorption light amount in a resist varies, or the reflectance on the surface of the resist varies.
In addition, the paper titled "Characteristics of Standing Wave Effect of Off-axis Illumination Depending on Two Different Resist Systems and the Polarization Effect of Stepper" in SPIE Optical/Laser Microlithography, VII, Vol. 2197, p 42, K. H. Kim, et al., demonstrates, based on experiments, that the swing curve obtained under oblique illumination differs from that obtained under normal illumination.
Thus, there has been a need for a calculation model of absorption light amount in a resist in consideration of an NA, a light source shape (coherence factor .sigma.), and diagonal incidence light on a wafer surface due to diffraction by a mask pattern.
On the other hand, the paper, titled "A general simulator for VLSI lithography and etching processes: Part 1--Application to projection lithography" in IEEE Trans. Electron Devices, Vol. ED-26, No. 12, p. 717, W. G. Oldham, et al., shows an example of a simulator for performing development calculations by conversion to a dissolving rate in a resist on the basis of a latent image distribution formed in the resist. A resist profile simulator such as a SAMPLE (Simulation and Modeling of Profiles for Lithography and Etching), described in this paper, is well known.
It is possible to find the swing curve by calculating a desired exposure condition and a resist line width obtained from a mask pattern, with use of the above resist profile simulator. However, in the above calculation, it is necessary to find the shape of the resist. Consequently, the calculation time is longer than in the conventional method. It is difficult to use this method, for example, in determining an optimal design of an anti-reflective layer to be formed on a semiconductor substrate.
When a resist pattern shape evaluation is performed with the aim of optical proximity correction (OPC) in consideration of even a multi-layer structure on a wafer, it is necessary to perform a shape evaluation calculation in a region of a certain width. Thus, when the shape evaluation calculation is performed after a latent image distribution in the resist is strictly found, the calculation time is too long.
On the other hand, in the case where a resist pattern shape evaluation calculation is performed on the basis of the amount of exposure light applied into the resist in proportion to a swing curve value by making use of a result obtained by a conventional swing curve calculation, an effective amount of exposure light applied to a pattern also varies due to each the patterns. As a result, it is not possible to exactly evaluate the resist pattern shape.
As has been described above, in the conventional swing curve calculation method in which only vertical incident light is considered, the influence of the use of an exposing apparatus with a high NA (numerical aperture), a modified illumination and a phase-shifting mask cannot be considered. On the other hand, in the method of calculating the resist pattern and finding the pattern dimensions from the resist pattern, the calculation time increases considerably.